je sais pas...
p't'etre que ca pourra t'aider...
<html>
<body>
<form name="monForm">
<select name="monSelect" onchange="(document.monForm.monSelect.options[document.monForm.monSelect.selectedIndex].text == 'Blabla')?document.monForm.monChamp.style.visibility = 'visible' ocument.monForm.monChamp.style.visibility = 'hidden';">
<option value="1" SELECTED><-------></option>
<option value="2">Blabla</option>
<option value="3">Bidulle</option>
</select>
<input type="text" value="LToPiQ c'est un gros nul en HTML" name="monChamp" style="visibility:hidden" size="40">
</form>
</body>
</html>